Function for 1-D plotting

I have to do quite a bit of 1-D plotting to explain local stability analysis. The code at http://trac.sagemath.org/sage_trac/ti... works but labeling specific points is close to impossible because of the way Sage adjusts the aspect ratio. So I put together a simple function that plots a line and two invisible points, allowing me to know where things are going to go (and making a long list of plot commands a bit neater). Here's the code.